[DOC] New homepage for docs.ruby-lang.org/en (ruby#12227)

.rdoc_options

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 page_dir: doc
 charset: UTF-8
 encoding: UTF-8
-main_page: README.md
+main_page: index.md
 title: Documentation for Ruby development version
 visibility: :private
 rdoc_include:

doc/index.md

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+# Ruby Documentation
+
+Welcome to the official Ruby programming language documentation.
+
+## Getting Started
+
+New to Ruby? Start with our [Getting Started Guide](https://www.ruby-lang.org/en/documentation/quickstart/).
+
+## Core Classes and Modules
+
+Explore the essential classes and modules:
+
+- [String](String.html) - Text manipulation and string utilities.
+- [Symbol](Symbol.html) - Named identifiers inside the Ruby interpreter.
+- [Array](Array.html) - Ordered collections of objects.
+- [Hash](Hash.html) - Key-value pairs for efficient data retrieval.
+- [Integer](Integer.html) - \Integer number class.
+- [Float](Float.html) - Floating-point number class.
+- [Enumerable](Enumerable.html) - Collection traversal and searching.
+- [File](File.html) - \File operations and handling.
+- [IO](IO.html) - Input/output functionality.
+- [Time](Time.html) - \Time representation.
+- [Regexp](Regexp.html) - Regular expressions for pattern matching.
+- [Range](Range.html) - Representing a range of values.
+- [Exception](Exception.html) - Base class for all exceptions.
+- [Thread](Thread.html) - Multithreading and concurrency.
+
+## Language Reference
+
+Deep dive into Ruby's syntax and features:
+
+- [Ruby Syntax](rdoc-ref:syntax.rdoc)
+- [Exceptions](rdoc-ref:exceptions.md)
+- [Implicit Conversions](rdoc-ref:implicit_conversion.rdoc)
+
+## Standard Libraries
+
+There are some standard libraries included in Ruby that are also commonly used, such as:
+
+- [Date](Date.html) - \Date representation.
+- [JSON](JSON.html) - \JSON encoding and decoding.
+- [ERB](ERB.html) - Embedded Ruby for templating.
+- [Net::HTTP](Net/HTTP.html) - HTTP client library.
+
+Use the following links to access the comprehensive set of libraries included with Ruby:
+
+- [Standard Library Documentation](rdoc-ref:standard_library.rdoc)
+- [Maintainers](rdoc-ref:maintainers.md)
+
+## Contribute to Ruby
+
+Get involved with the Ruby community:
+
+- [Contribution Guide](rdoc-ref:contributing.md)
+- [Documentation Guide](rdoc-ref:contributing/documentation_guide.md)
+- [Reporting Issues](rdoc-ref:contributing/reporting_issues.md)
+- [Building Ruby](rdoc-ref:contributing/building_ruby.md)
+- [Testing Ruby](rdoc-ref:contributing/testing_ruby.md)
+- [Issue Tracker](https://bugs.ruby-lang.org/projects/ruby-master/issues)
+
+## Additional Resources
+
+- [Ruby Homepage](https://www.ruby-lang.org/)
+- [RubyGems](https://rubygems.org/)
+- [Ruby Community](https://www.ruby-lang.org/en/community/)